Heat-developable photosensitive materials having a heat-developable photosensitive layer containing at least an organic silver salt, a photocatalyst and a reducing agent have been described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075. These heat-developable photosensitive materials have advantages such as excellent gradation and high sensitivity as compared with other dry-type photographic processes, such as an electrophotographic process, a diazo photographic process, a foaming photographic process, or a heat-sensitive photographic process utilizing a coloring reaction of a leuco dye. In these heat-developable photosensitive materials, the image is obtained by imagewise exposure to light so that said photocatalyst is able to catalyze an oxidation-reduction reaction of said organic silver salt with the reducing agent, and thereafter heating (generally to 80.degree. C. or more, and preferably to 120.degree. C. or more). In this photosensitive system, since the photocatalyst having photosensitivity is used in a very small amount, there is hardly any interference when viewing, even if the sensitive material is allowed to remain exposed to light without carrying out fixation or stabilization after the development.
However, one of the faults of these heat-developable photosensitive materials is that the image becomes blurred by halation. In the prior silver halide photographic materials, which are subjected to a wet processing, this problem has been overcome by providing an antihalation layer, which is bleached or removed by liquid processing after exposure, located on the back of the base or between the base and the emulsion layer. However, an antihalation layer which is bleached or removed by liquid processing is not useful for heat-developable photosensitive materials, because the heat-developable photosensitive materials are intended to be processed by heating only.
Accordingly, in order to prevent halation of the heat-developable photosensitive materials, it is necessary to use an antihalation layer which can be bleached or removed by a dry processing. Techniques proposed heretofore include the following:
(1) A method which comprises using a colored layer capable of being mechanically stripped as the antihalation layer, as described in Japanese Patent Publication No. 33692/73.
(2) A method which comprises using a colored layer containing a dye which is bleached by heat as the antihalation layer, as described in U.S. Pat. Nos. 3,769,019, 4,033,948, 4,088,497, 4,153,463 and 3,821,001 and Japanese Patent Application (OPI) No. 56818/79 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
(3) A method which comprises using a colored layer containing o-nitro-arylidene dyes or o-nitro-o-azaarylidene dyes, which are bleached by light, as the anti-halation layer, as described in Japanese Patent Application (OPI) No. 17833/79.
However, the method (1) requires an additional processing apparatus for the sole purpose of removing the antihalation layer, and there is also the possibility of damaging the base during the stripping operation. On the other hand, in method (2), there is a possibility of causing bleaching before the processing or during preservation, depending on the temperature, or there is a possibility of causing excess heat fogging. Further, in the method (3), there is a possibility of causing light discoloration of the heat-developable photosensitive layer because a large quantity of light is required for the decoloring, and there is a problem of selection of appropriate dyes having a desired absorptive wavelength, and synthesis of such dyes are difficult. Furthermore, the most serious disadvantage of the methods (2) and (3) is that decoloration of the colored antihalation layer requires a long period of time, i.e., the decoloration rate of the colored antihalation layer is low.